Just in case you ever worry about being too out of character, I'll keep things here that may help you along the road of RPing. If you have anything else to add, let me know.

This is meant for fanfics, but it works just as good for RPing. Although, I do have to admit, I can probably disagree with maybe one or two of these.

Check out descriptions of your characters. Even if they are small descriptions, basic information is always nice.

Read a couple manga pages involving your RP characters! Strips that weren't involved in the anime are usually most helpful, so you can get to know more reactions of a character, or you can go back and read ones that were in the anime too- after all, there are differences in both the anime and manga.

Learn about pairings when wanting to RP them. This only mentions popular pairings though, lesser known ones would have to have other research done on them. However, make sure that the other RPer is willing to play along the pairing with you.

Besides that, I would suggest checking the Hetalia wiki (which I'm going to assume most of you have before reading all this) every so often, which is a good place to start when looking for a character's personality and their relationships to other characters. If not, whenever you're in doubt, you can always search around and ask others for their opinions on your RP skills for a certain character.